The present invention pertains to an inductive load driving circuit for supplying current to an inductive load. In particular, the present invention pertains to a technology for preventing occurrence of surge voltage caused by the counterelectromotive force of the inductive load.
Conventionally, an inductive load supply circuit has been used widely to supply current to an inductive load, such as the winding in a motor.
Its operation theory will be explained using inductive load driving circuit 101 shown in FIG. 6.
Said inductive load driving circuit 101 has an output transistor 105 made of an n-channel MOSFET, a transistor control circuit 118 that operates said output transistor 105, and an inverter 112 that is inserted between the output terminal of transistor control circuit 118 and the gate terminal of output transistor 105 to invert the output signal of transistor control circuit 118 and then send it to the gate terminal.
Symbol 121 represents the output terminal on the high potential side of said inductive load driving circuit 101, while symbol 122 represents the output terminal on the low potential side. A positive voltage output from DC voltage source 123 is applied to the high-potential output terminal 121. The drain terminal of output transistor 105 is connected to the low-potential output terminal 122.
A load 126 is connected between high-potential output terminal 121 and low-potential output terminal 122. When output transistor 105 is turned on and low-potential output terminal 122 is grounded, a current is supplied from DC voltage source 123 to load 126.
When output transistor 105 is converted from on to off, the current flowing to load 126 is stopped.
Since the current flowing to load 126 can be controlled with the on and off state of output transistor 105 as described above, the magnitude of the average current flowing to load 126 can be maintained at a constant level by keeping the on period and off period of output transistor 105 as well as their ratio constant.
However, since load 126 is inductive, an induced electromotive force with positive polarity is generated at low-potential output terminal 122 when output transistor 105 is converted from the off state to the on state.
FIG. 7 shows the variation in the voltage of low-potential output terminal 122. The peak near 50 xcexcsec in the diagram is a surge voltage caused by the induced electromotive force. Since the surge voltage will damage output transistor 105 and cause malfunction of other circuits, it is necessary to use a different diode outside inductive load driving circuit 101 to absorb the surge voltage. As a result, the cost will be increased, and it becomes difficult to miniaturize the circuit.
The objective of the present invention is to solve the aforementioned problems by providing an inductive load driving circuit that generates no surge voltage.
In order to realize the aforementioned objective, the present invention provides an inductive load driving circuit having a main transistor for supplying a current path to an inductive load, an auxiliary transistor that is connected in parallel with the aforementioned main transistor, and a control circuit, which has a first driver that supplies a first control signal to the control terminal of the aforementioned main transistor, a first wave shaping circuit that blunts the rising and falling characteristics of the aforementioned first control signal, a second driver that supplies a second control signal to the aforementioned auxiliary transistor, and a second wave shaping circuit that blunts the rising and falling characteristics of the aforementioned second control signal, and which can turn on/off both the aforementioned main transistor and auxiliary transistor.
In the inductive load driving circuit of the present invention, the aforementioned first wave shaping circuit has a first capacitor connected to the control terminal of the aforementioned main transistor, while the second wave shaping circuit has a second capacitor connected to the control terminal of the aforementioned auxiliary transistor.
Also, in the inductive load driving circuit of the present invention, the aforementioned first wave shaping circuit has a first resistor for restricting current to blunt the waveform of the aforementioned first control signal when turning off the aforementioned main transistor, and the aforementioned second wave shaping circuit has a second resistor for restricting current to blunt the waveform of the aforementioned second control signal when turning off the aforementioned auxiliary transistor.
In the inductive load driving circuit of the present invention, the aforementioned main transistor consists of an n-channel MOS transistor, while the aforementioned auxiliary transistor consists of a p-channel MOS transistor.
In addition, in the inductive load driving circuit of the present invention, the on/off timing of the aforementioned main transistor and auxiliary transistor are staggered.
In the aforementioned configuration, since the off timing of the auxiliary transistor is staggered, for example, delayed from the off timing of the main (output) transistor, the auxiliary transistor can release the energy remaining in the inductive load.
Also, when the source terminal of the auxiliary transistor is connected to the terminal where the counterelectromotive force of the inductive load occurs, even if a counterelectromotive force occurs in the inductive load when the output transistor is turned off, the magnitude of the counterelectromotive force can be clamped by the threshold voltage between the source terminal and gate terminal of the auxiliary transistor.
Also, the output transistor can be turned off slowly, and the current flowing to the output transistor is reduced gradually. After the current flowing to the inductive load is reduced to a level that will not cause a counterelectromotive force, the auxiliary transistor is turned off.